Editing Compound steam engine (section)

===Compounding engines===
A solution to the dilemma was invented in 1804 by British engineer [[Arthur Woolf]], who patented his ''Woolf high pressure compound engine'' in 1805. In the compound engine, high-pressure steam from the boiler first expands in a high-pressure (HP) cylinder and then enters one or more subsequent lower pressure (LP) cylinders. The complete expansion of the steam occurs across multiple cylinders and, as there is less expansion in each cylinder, the steam cools less in each cylinder, making higher expansion ratios practical and increasing the efficiency of the engine.

There are other advantages: as the temperature range is smaller, cylinder condensation is reduced. Loss due to condensation is restricted to the LP cylinder. Pressure difference is less in each cylinder so there is less steam leakage at the piston and valves. The [[torque|turning moment]] is more uniform, so balancing is easier and a smaller flywheel may be used. Only the smaller HP cylinder needs to be built to withstand the highest pressure, which reduces the overall weight. Similarly, components are subject to less strain, so they can be lighter. The reciprocating parts of the engine are lighter, reducing the engine vibrations. The compound could be started at any point in the cycle, and in the event of mechanical failure the compound could be reset to act as a simple, and thus keep running.<ref name=Raiput/>

To derive equal work from lower-pressure steam requires a larger cylinder volume as this steam occupies a greater volume. Therefore, the bore, and in rare cases the stroke as well, are increased in low-pressure cylinders, resulting in larger cylinders.

Double-expansion (usually just known as 'compound') engines expand the steam in two stages, but this does not imply that all such engines have two cylinders. They may have four cylinders working as two LP-HP pairs, or the work of the large LP cylinder can be split across two smaller cylinders, with one HP cylinder exhausting into either LP cylinder, giving a 3-cylinder layout where the cylinder and piston diameter of all three are about the same, making the reciprocating masses easier to balance.

Two-cylinder compounds can be arranged as:
* '''Cross-compound''' – the cylinders are side by side
* '''Tandem compound''' – the cylinders are end to end, driving a common connecting rod
* '''Telescopic-compound''' – the cylinders are one inside the other
* '''Angle-compound''' – the cylinders are arranged in a vee (usually at a 90° angle) and drive a common crank.{{cref|Cylinder phasing}}<!--Not sure what name is best for this note link, nor where best to put it-->

The adoption of compounding was widespread for stationary industrial units where the need was for increased power at decreasing cost, and almost universal for [[marine steam engine|marine engines]] after 1880. It was not widely used in railway locomotives where it was often perceived as complicated and unsuitable for the harsh railway operating environment and limited space afforded by the [[loading gauge]] (particularly in Britain). Compounding was never common on British railways<!--small 'r'!--> and not employed at all after 1930, but was used in a limited way in many other countries.{{sfn|Van Riemsdijk|1994|pages=2–3 }}

The first successful attempt to fly a heavier-than-air fixed-wing aircraft solely on steam power occurred in 1933, when [[Doble steam car#Besler Brothers|George and William Besler]] converted a [[Travel Air 2000]] biplane to fly on a 150&nbsp;hp angle-compound V-twin steam engine of their own design instead of the usual [[Curtiss OX-5]] inline or radial aviation gasoline engine it would have normally used.<ref>[https://books.google.com/books?id=AygDAAAAMBAJ&pg=PA9 "World's First Steam Driven Airplane"] ''Popular Science'', July 1933, detailed article with drawings</ref><ref>{{Cite video
  | people = George & William Besler
  | title = The Besler Steam Plane 
  | medium = YouTube 
  | publisher = Bomberguy
  | url = https://www.youtube.com/watch?v=nw6NFmcnW-8 
  | date = 29 April 2011}}</ref>